The present invention is adapted for use with a conventional drive system in which a driven pulley and a drive pulley are connected by a flexible drive element such as a drive belt. In such an arrangement, it is well known that the proper functioning of the drive system and the operating life of the drive belt can be significantly enhanced by maintaining the two drive belt runs between the pulleys in tension such that significant slack does not occur in either run. One method of providing such tension is to resiliently bias the pulleys away from one another. However, a more convenient arrangement is provided by a tensioning device that biases one or both drive belt runs inwardly towards the other run at a point intermediate the pulleys. This arrangement provides the added advantage of keeping a greater surface area of the drive belt in contact with the outer surface of the pulleys at any given period of time. A number of examples of this latter type of tensioning device are disclosed in the prior art. However, in the great majority of prior tensioning devices, the tensioning device is adapted to operate in a particular drive system, e.g., to operate with a given arrangement of pulleys and drive belt.
One result of the fact that prior tensioning devices have been adapted for specific applications is that most such devices have been mounted or secured to a support that is fixed with respect to the pulley axles. Through use of such a support arrangement, the position of the tensioning device between the pulleys cannot be controlled, to maintain the tensioning device at the optimum position. The use of fixed supports has conventionally been viewed as especially important for tensioning devices adapted to operate with pulleys that are or may be horizontally positioned with respect to one another. In such a horizontal arrangement, the weight of the tensioning device does not affect its position, i.e., the position of the tensioning device between the pulleys is not in any way controlled by gravity acting on the tensioning device.
A further feature of the majority of prior art tensioning devices is that in such devices, the surfaces contacting the two runs of the drive belt and urging them inward are either fixed in position with respect to one another, or resiliently biased towards one another by springs or similar means. A disadvantage of the resilient biasing technique is that it adds complexity to the tensioning device, and the spring or other resilient means are themselves subject to wear over time. Tensioning devices having fixed distances between their contact surfaces cannot be adjusted as the belt experiences increased wear. Thus, the tensioning force provided by the device decreases over time, at least in those arrangements where the pulleys are horizontally positioned with respect to one another. A further disadvantage of a fixed distance between the contact surfaces is that the tensioning device cannot be used with different sized pulleys.
One attempt to overcome the drawbacks of the prior art has been provided by the belt tensioner disclosed in U.S. Pat. No. 4,068,535 to Sheets. This device utilizes upper and lower idler assemblies, each of which carries significantly longitudinally spaced rollers, which are adjustably connected by threaded rods. However, this device is not easily adjusted, due to the need to undo locking washers and nuts, and is not compact, due to the widely spaced rollers. Thus, this device is not well suited for use on drive belts which operate in closely confined spaces.
Additional partial solutions for tensioning drive mechanism tensioners are provided by the chain tensioners disclosed in U.S. Pat. No. 4,662,862 to Matson, U.S. Pat. No. 4,798,562 to Matson et al. and U.S. Pat. No. 4,850,934 to Gibson Jr. et al. These devices provide upper and lower low friction drive chain contact members that glide on chain surfaces, and which are incremented by adjustable straps. The straps are readily adjusted to change the spacing of the contact members, particularly in a disclosed ratcheting strap version. However, these devices, while well suited for use on chains, are not as well suited for use on belts due to the increased friction associated with belt contact on the contact members.
Further drawbacks of prior art tensioning devices stem from their lack of adjustability. As has been previously mentioned, it is advantageous to be able to adjust a tensioning device such that the surfaces of the tensioning device, which contact the two runs of a flexible drive element, can be positioned closer to one another. Additionally, in tensioning devices that contain more than one contact surface for each run of a flexible drive element, it is also advantageous to be able to position the contact surfaces on each side of the flexible drive element closer or further away from one another. It is a further benefit for a tensioning device to be of a configuration that adequately supports the contact surfaces in a stable arrangement. It is of still further benefit for a tensioning device to be able to manage multiple drive elements and relatively long drive elements, both of which have their own specific problems, such as the tendency for the runs to tangle with one another and/or to flip an associated tensioning device.